4.3. Spatial Variations of Stellar Populations
Not surprisingly properties such as gas and stellar content, age structure, metallicity distribution, density, and scale height vary as a function of position within a galaxy. Spatial variations in the distribution of stellar populations of different ages are found in all types of galaxies, underlining the importance of large-area coverage when trying to determine the star formation history of a galaxy.
The oldest populations turn out to be spatially most extended. Spiral galaxies in the Local Group show pronounced population differences between disk, halo, and more intricate spatially and kinematically distinct subdivisions. In massive irregulars such as the LMC spatial variations are traced by, e.g., multiple distinct regions of concurrent star formation. These regions can remain active for several 100 Myr, are found throughout the main body of these galaxies, and can migrate.
In low-mass dIrrs and several dSphs the most recent star formation events are usually centrally concentrated. A radial age gradient may be accompanied by a radial metallicity gradient, indicating that not only gas but also metals were retained over an extended period of time. Occasionally evidence for shell-like propagation of star formation from the central to adjacent regions is found. DSphs that are predominantly old tend to exhibit radial gradients in their HB morphology such that the ratio of red to blue HB stars decreases towards the outer parts of the dwarfs. If such second-parameter variations are caused by age then this would indicate star formation persisted over a longer period of time in the centers of these ancient galaxies.